Conventional electrical power systems utilize a synchronous electrical generator for generating AC power. Particularly, such a generator may include a rotor and a stator having a stator coil. In applications such as an aircraft, the rotor is driven by an engine so that electrical power is developed in the stator coil. Owing to the variation in engine speed, the frequency of the power developed in the generator windings is similarly variable. This variable frequency power is converted to constant frequency power using a variable speed constant frequency (VSCF) system including a power converter which may develop, for example, 115/200 V.sub.ac power at 400 Hz. Such known converters are controlled by a generator/converter control unit (GCCU).
In order to provide aircraft engine starting, such known power systems have operated the generator as a motor. Specifically, an external power source is coupled through a start control to the generator to energize the stator coil and thus develop motive power to start the engine. The components required in such a start control increase the weight of the aircraft and take up valuable space. To minimize the size and weight of such start controls, certain known aircraft VSCF power systems have utilized the existing converter and GCCU for the start control.
In the start mode of operation, the converter may be supplied power from any 400 Hz power source, such as, for example, an auxiliary power unit generator or an external power source. However, each such power source might have a different available capacity for use in engine starting. Therefore, the GCCU must be configured to provide engine starting from any such available power sources and to limit the amount of power drawn.
Rozman et al. copending application No. 270,625 entitled "VSCF Start System With Selectable Input Power Limiting", assigned to the assignee of the present invention, discloses a start control system for a brushless DC machine which is operable to maintain constant motor current and thus control input power. Specifically, as disclosed therein, a start system is used for starting an engine using a brushless synchronous generator operating as a motor. The motor receives power from a main inverter and an excitation inverter. These inverters are controlled by a GCCU which provides for current control and constant power characteristics in the field weakening range. The control unit includes a pulse width modulation (PWM) generator which is responsive to a voltage command and a commutation angle command to develop switching signals for controlling the switches in the main inverter. The voltage command has a level corresponding to rotor speed plus a boost voltage to offset the IR drop of the machine at low speeds. In fact, the voltage command is selected to maintain a constant volt/hertz ratio. The voltage command is used to vary the duty cycle of the PWM signals.
The solution described in the copending Rozman et al. application utilized open loop voltage control. Motor armature current is controlled by varying the commutation angle. Such an arrangement limits the range of motor armature current control, specifically at low speed.
The present invention is intended to overcome one or more of the problems as set forth above.